X-rays illuminate the interior of the Moon

Contrary to Earth, our Moon has no active volcanoes, and the traces of its past volcanic activity date from billions of years ago. Scientists have now identified a likely reason for this peaceful surface life: the hot, molten rock in the Moon's deep interior could be so dense that it is simply too heavy to rise to the surface like a bubble in water. For their experiments, the scientists produced microscopic copies of moon rock collected by the Apollo missions and melted them at the extremely high pressures and temperatures found inside the Moon. They then measured their densities with powerful X-ray beams. The results are published in 'Nature Geoscience'.

The team was led by Mirjam van Kan Parker and Wim van Westrenen from Vu University Amsterdam and comprised of scientists from the Universities of Paris, Lyon and Edinburgh, from Cnrs and the European Synchrotron Radiation Facility (Esrf) in Grenoble."We had to use the most brilliant X-ray beam in the world for this experiment because the magma sample is so tiny and confined in a massive, highly absorbing container. Without a bright beam of X-rays, you cannot measure these density variations", says Mohamed Mezouar from the Esrf.

The measurements at the Esrf were combined with computer simulations to calculate the magma density at any location in the Moon. Nearly all the lunar magmas were found to be less dense than their solid surroundings, similar to the situation on Earth. There is one important exception: small droplets of titanium-rich glass first found in Apollo 14 mission samples produce liquid magma as dense as the rocks found in the deepest parts of the lunar mantle today. This magma would not move towards the surface. Such titanium-rich magma can only be formed by melting titanium rich solid rocks. Previous experiments have shown that such rocks were formed soon after the formation of the Moon at shallow levels, close to the surface. How did they get deep into the mantle?

The scientists conclude that large vertical movements must have occurred early in the history of the Moon, during which titanium-rich rocks descended from near the surface all the way to the core-mantle boundary. "After descending, magma formed from these near-surface rocks, very rich in titanium, and accumulated at the bottom of the mantle - a bit like an upside-down volcano", concludes van Westrenen. "Today, the Moon is still cooling down, as are the melts in its interior. In the distant future, the cooler and therefore solidifying melt will change in composition, likely making it less dense than its surroundings. This lighter magma could make its way again up to the surface forming an active volcano on the Moon - what a sight that would be! But for the time being, this is just a hypothesis to stimulate more experiments".